Supercapacitor and a method of manufacturing such a supercapacitor

ABSTRACT

An electric double layer capacitor including at least one pair of polarizable electrodes connected to current collectors, a separator made of ion-permeable but electron-insulating material interposed between the electrodes in each pair of electrodes, and a liquid electrolyte. According to the invention the electrodes include a layer of carbon particles having a narrow distribution of nanopores therein, the pore sizes of the nanopores being adapted to fit the ion sizes of the electrolyte. The invention also relates to a method of manufacturing such a supercapacitor.

FIELD OF THE INVENTION

[0001] This invention relates in general to the field ofsupercapacitors. More particularly, this invention relates to a superiorsupercapacitor having electrodes fabricated from specially synthesizednanostructured carbon (SNC) powders in which the pore sizes and thespecific surface may be selectively controlled.

BACKGROUND OF THE INVENTION

[0002] Electric double layer capacitors are widely used in a variety ofindustries. For example, PCT WO99/24995 shows an electric double layercapacitor and manufacturing method. The electric double layer capacitorcomprises metal foil or mesh current collectors, polarizable electrodesmade of an activated carbon and a polymer binder, conductive coatingscomprising a melamine resin binder at the interfaces between currentcollectors and polarizable electrodes, and non-aqueous electrolyte. Thethickness of the polarizable electrodes is 15 micron. The electrodeswere deposited into the conductive layer coated current collector by ascreen printing method. Such a electric double layer capacitordemonstrates long term low impedance at the electrode-current collectorinterface and high power capabilities.

[0003] Another prior art capacitor is described in U.S. Pat. No.5,150,283, showing an electric double layer capacitor and method forproducing the same. The method includes depositing a mixture prepared bydispersing activated carbon powder and agent for improving the electricconductivity of the layer on an aluminum substrate by either means ofspreading, impregnating or printing. The layer thickness is equal to50-100 micron.

[0004] U.S. Pat. No. 5,776,633 describes carbon/carbon compositematerials and use thereof in electrochemical cells. This inventionprovides an activated carbon fabric impregnated with a mixture ofactivated carbon powder and binder; the thickness of materials obtainedbeing 125-250 micron. The advantage of materials obtained includes thelow resistivity, the ability to easily form thin composites with verygood mechanical strength.

[0005] Another example of an electric double layer capacitor isdescribed in U.S. Pat. No. 5,142,451. Specifically, an electric doublelayer capacitor which comprises a plurality of thin plate-like electrodeelements is disclosed. The polarizable electrodes were manufactured bysintering powder of minute active carbon particles having an averagediameter of about 20 micron into a porous sintered electrode body. Thispolarizable electrode is held in contact with a current collectorthrough an electrically conductive layer to reduce the inner resistanceof the capacitor.

[0006] A method for manufacturing a polarizable electrode for electricdouble layer capacitor is taught in U.S. Pat. No. 5,277,729. Thethickness of electrode is about 40-500 micron. The electrode is obtainedby hot rolling an initial mixture of fine carbon powder, polymer resinand liquid lubricant.

[0007] A metal electrode material, capacitor using metal electrodematerial, and method of manufacture is disclosed in PCT WO 99/38177. Themetal electrode material comprises a “valve metal material” with carbonparticles on its surface. The metal electrode material is coated with anactivated carbon layer and used as polarizable electrode for an electricdouble layer capacitor. That capacitor has reduced inner resistance,resulting in an increase in capacitance.

[0008] U.S. Pat. No. 5,742,474 describes an electric double layercapacitor, comprising a pair of polarized electrodes made of the sameactivated carbon materials. However, the amount of the carbon materialof the polarized positive electrode is higher than that of the polarizednegative electrode. The main advantage is that it is possible toincrease a voltage applied to the electric double layer capacitor.

[0009] The vast majority of carbon materials used for electrodes inelectric double layer capacitors (EDLC) have been prepared by thecharring or carbonization of organic substances, usually followed by asurface activation process using water vapor or other activation agent.

[0010] The foregoing demonstrates that electrodes and capacitors havebeen widely studied in the prior art. Yet with all of this study, thereis still a great need for the development of supercapacitors thatexhibit superior performance.

DEFINITIONS

[0011] For the purpose of this patent application, the terms nanoporous,nanoporosity and nanostructured apply to pore sizes less than 3nanometer. By transport porosity is meant pores larger than 3nanometers.

SUMMARY OF THE INVENTION

[0012] In summary, an object of the present invention is to provide asupercapacitor, which exhibits superior performance. In particular, thepresent invention provides the following features. Specially synthesizednanostructured carbon (SNC) powder is processed to fabricate electrodesin such a manner that the resultant electrodes have pore sizes, whichare selectively and closely controlled. Further, thin, compositealuminum and SNC electrodes are made using the carbon powder.Additionally, by the ability to selectively control the resultant poresizes in the electrodes, a capacitor is provided wherein the positiveand negative electrodes are balanced with respect to their nanopore sizeand active carbon content, thereby tailoring the electrodes to fit theionic sizes of the electrolyte positive and negative ions respectivelyemployed with the capacitor. This also allows, in another aspect of thepresent invention, the selection of the most efficient electrolyte withrespect to its conductivity and other desirable features. The presentinvention also provides for the selection of a desirable separator,which gives increased conductivity and leaves sufficient free ionconcentration when charged.

[0013] In another aspect of the present invention, a method is providedwherein SNC is synthesized from inorganic polycrystalline material toselectively control the pore size and pore size distribution in theresulting electrode.

[0014] In one embodiment, an electric double layer capacitor is providedcomprising: at least two thin and flexible polarizable electrodesobtained by rolling a mixture of SNC material with a binder. Saidelectrodes are connected to metal current collectors; a thin layer of aporous, ion-permeable but electron-insulating material (separator)interposed between electrodes; and a liquid electrolyte.

[0015] In another embodiment, an electric double layer capacitor isprovided comprising: a pair o f polarizable electrodes made of a SNCmaterial having different porosities (and pore size distributions); athin layer of a porous, ion-permeable but electron-insulating material(separator) interposed between the electrodes; and a liquid electrolyte.

BRIEF DESCRIPTION OF THE TABLES AND FIGURES

[0016] TABLE 1 is a table showing pore structure parameters for SNC(SiC)powder modified by HNO₃.

[0017] TABLE 2 is a comparison of electrochemical behaviour ofnon-modified and modified SNC materials.

[0018] TABLE 3 shows properties of modified nanostructured carbon fromdifferent precursors.

[0019] TABLE 4 gives electrochemical characteristics of some compoundsselected as voltage equalizing additives.

[0020] TABLE 5 illustrates the influence of carbon material from variousprecursor material on capacitance in electric double layer capacitorswith water based electrolyte systems.

[0021] TABLE 6 shows results of different methods of connecting analuminum current collector to a carbonaceous electrode sheet.

[0022] TABLE 7 is a rendering of the effect of balancing positively andnegatively charged polarizable electrodes.

[0023] TABLE 8 gives examples of electrochemical performance ofprototype electric double layer capacitors according to the presentinvention.

[0024]FIG. 1 is s schematic drawing of supercapacitor device comprising4 anodes and 4 cathodes connected in parallel according to the presentinvention.

[0025]FIG. 2 is a graph of the pore size distribution of SNC(TiC)

[0026]FIG. 3 is a graph of the pore size distibution of SNC(Mo₂C)

[0027]FIG. 4 is a graph of the pore size distribution of SNC((MoTi)Cx)

[0028]FIG. 5 compares the pore size distribution of carbon powders fromTiC before and after modification

[0029]FIG. 6 is a Ragone plot of specific energy and specific power ofan unpacked electrochemical system of device numbe 1 in table 8.

[0030]FIG. 7 shows some electrolytic salts (cation and anions) used inelectrolytes for electric double layer capacitors.

DETAILED DESCRIPTION OF THE INVENTION

[0031] The invention will now be described in more detail with referenceto exemplifying embodiments thereof and also with reference to theaccompanying drawings of which FIG. 1 illustrates a side view of acapacitor with 4 anodes and 4 cathodes connected in parallell inaccordance with the present invention. The capacitor with doubleelectric layers generally includes a hermetic case 5, closed by asealing 7. Inside of the case are situated one or more electrode pairsof which 1 is the anode and 2 is the cathode. The electrodes 1 and 2 aresaturated with an electrolyte and separated by means of a porousseparator 4. To the electrodes 1 and 2 are attached metal currentcollerctors 3 which are in turn joined to a terminal lead 6.

[0032] The present invention provides in one aspect SNC powder uniquelymanufactured with closely controlled pore sizes. SNC powder is processedto fabricate electrodes in such a manner that the resultant electrodehas pore sizes which are selectively and closely controlled. Further,thin electrodes are made using the carbon powder. In another aspect ofthe present invention by selectively controlling the resultant pore sizein the electrodes, a capacitor is provided wherein the positive andnegative electrodes are balanced with respect to their nanopore size andactive carbon content. This allows one to tailor the electrodes to fitthe ionic sizes of the positive and negative electrolyte ions employedwith the capacitor. This also provides, in another aspect of the presentinvention, the selection of the most efficient electrolyte with respectto its conductivity and other desirable features. In yet another aspectof the present invention, the selection of a desirable separator isprovided, which possesses desirable ionic conductivity and leavessufficient free ion concentration when charged.

[0033] Additionally, a method is provided wherein SNC is synthesized toselectively control the pore size and pore size distribution in theresulting electrode. Finally, a post treatment of the nanoporous carbonmaterial for fine tuning of the pore size and its distribution isintroduced.

[0034] More particularly, SNC materials are produced by thermo-chemicaltreatment of carbides or related carbon-containing compounds. The choiceof carbon containing compound and respective synthesis conditionscontrols the size of nanopores and the extent of the active surfacearea. The ability to control the pore size and porosity is required tomatch the ion sizes in different electrolytes used in the supercapacitorapplication.

[0035] Process for a preparation of mineral active carbons from metal ormetalloid carbides and some carbonitrides is described in GB 971943 thatwas focused on producing activated carbon powders, which would havesuperior adsorption behaviors. Major difference of the present inventionfrom the prior art mentioned above is to provide the carbonaceousmaterial for electric double layer capacitors having large capacitanceper volume and low electrical resistivity. These targets are achievableby a material of high density in the sense that no wasteful porosityoccurs. Another aspect of this invention is to provide the process forproducing such a carbonaceous material.

[0036] There is a large range of possible carbon containing precursorcompounds of the general formula MCON, where M is a metal, C is carbon,O is oxygen and N is nitrogen. Some of these compounds are more suitableto our process than others. For example if M is a transition metal suchas Titanium, then the simplest compound is TiCx where x is in the rangeof about 0.5-1. For TiC, the pore size of the resulting carbon has beenmeasured to have a peak value of about 0.6-0.8 nm. For TiC_(0.5) thepeak pore size of the resulting carbon is approximately 2.8 nm. Thecontrol of C sub-stoichiometry offers a method to control the nanoporesize resulting from halogenation. Substituting O and N for carbon inTiCx can further lower the total carbon content of the precursorcompound. This gives a further possibility to widen and control thetotal porosity and range of pore sizes. Substitutions for Titanium suchas with Molybdenum may also be made. In this case the Mo atomssubstitute for Ti in the same sintered metal lattice to form a solidsolution compound such as (TiMo)Cx. Upon halogenation, such a solidsolution yields a carbon with a nanopore size and distribution, whichdiffers beneficially from that of TiCx or MoCx.

[0037] FIGS. 2 to 4 show the pore distribution for TiC—, Mo₂C— andsintered (TiMo)Cx— derived carbon respectively.

[0038] According to one aspect of the present invention, carboncontaining compounds based on the following metals, or metalloids orcombinations thereof, are preferred be used: Ti, Zr, Hf, V, Nb, Ta, Mo,W, Cr, Fe, Al, Si, B, and Ca.

[0039] The size of the particles of the carbon precursor shouldpreferably be less than about 100 microns to provide good conditions forhalogenation throughout the particles. In most carbides, a particle sizeof less than about 10 microns is advantageous to avoid overheatinginside the particles, which during chlorination increases the amount ofundesirable graphitic clusters and closed porosity. Halogenation can bemade with all halogens, but Cl₂ is preferred. In its simplest form acharge of TiC powder is placed in a tube furnace heated to a desiredreaction temperature in the flow of inert gas. Thereupon Cl₂ gas ispassed through the powder mass and allowed to react until all Titaniumis removed from the carbide. The mass balance of the reaction can bepresented as:

TiC+2Cl₂πTiCl₄+C

[0040] The TiCl₄ being a vapor at the reaction temperature is swept awayand condensed in a collector thus providing separation of the productsof reaction. In most cases the reaction of carbides with chlorine isexothermic and can increase the local temperature within the powdermass. The actual reaction

[0041] temperature affects the nanoscale structure of the carbon productand has to be kept below the temperature of graphitization. Forinstance, in the case of TiC as precursor material, 900-1000° C. ispreferred. Thus it is preferred to carry out chlorination underconditions of near uniform heat transfer, such as in a fluidized bed ora rotary kiln reactor. Carbides that form gaseous chlorine are preferredbecause their chlorides are vapors. The carbon product is subsequentlyheat treated at 900-1100° C. in the atmosphere of inert gas such asArgon or Helium to remove the excess of chlorine adsorbed in pores(dechlorination). Removal of any undesired residual chlorine includingchemically bound is done by additional heat treatment in preferably H₂atmosphere at 600-900° C. Diluted H₂O vapor at 800-1000° C. using inertcarrier gas, e.g. Argon, also works.

[0042] Even though the above method of manufacturing nanoporous carbongives good control of the size and size distribution of nanopores in theresulting particles, some additional refinement of the controllabilityis desirable. The match of nanopores size and ion size under a given setof circumstances, such as different size of cations and anions and theirdiffusive mobility inside the pores, is important; the pores should notbe too small or they will not be entered by the ions or if the fit istoo close, the mobility of the ions will be impeded.

[0043] On the other hand, if the nanopores are unnecessarily large, thespecific surface of the carbon material suffers. Certain carbideprecursors are more expensive than others and the carbon yield from theprocess also differs. For economic reasons as well as from functional,it is advantageous to apply a nanopores modification process of thecarbon realized by halogenation of the precusor material. This is doneby essentially known methods such as exposing at elevated temperaturethe nanoporous carbon to an oxidizing medium which may consist of H₂Ocarried by an inert gas, carbon dioxide, concentrated nitric orsulphuric acid or other oxidizing gents. The effects are controllablewidening of the nanopores and removal of physically and chemicallyabsorbed chlorine. In most cases it is preferable to use such modifiednanostuctured carbon for at least one of he electrodes in a electricdouble layer capacitor. FIG. 5 shows the effect on TiC derived carbon ofsuch modification. Table 1 shows the effect of halogenated SiC,subsequently modified by HNO₃. Table 2 shows electrochemical effects ofmodification of SiC and TiC derived carbon. Table 3 illustrates materialproperties of a number of modified carbon materials from variousprecursors. TABLE 1 Pore Structure Parameters for SNC (SiC) powdermodified by HNO₃ Pore size^(b) Surface area S, Pore volume Vp, X*, nmCycle number m²/g^(a) ccm/g (calculation) initial powder 1330 0.49 0.741 1420 0.55 0.78 2 1320 0.58 0.88 3 1260 0.65 1.03 4 1240 0.65 1.04

[0044] TABLE 2 Electrochemical comparison of non-modified and modifiedSNC materials S_(a,BET) [m²g⁻¹] Specific Capacitance SNC Type of elec-[F g⁻¹]* precursor SNC powder trode DC = −1.4V DC = −1.4V SiC Non- 1086931 5.8 79.9 modified Modified 2140 1567 92.8 88.1 (H₂O) TiC Non- 14851054 80.5 113.4 modified Modified 2232 1639 111.2 142.5 (H₂O)

[0045] TABLE 3 Examples of material properties of modifiednanostructured carbon from different precursors. Post- S_(a)(B W _(s)SSC Precursor T_(chlor) treatment ET) V_(p(total)) V_(p(nano)) (C₆H₆)C(+)^(a) C(−)^(a) # carbide ° C. agents m²/g cm³/g cm³/g cm³/g F/g F/g 1TiC 950 H₂ 1500 0.74 0.60 0.66 113 98 2 Mo₂C 750 H₂ 2138 1.59 0.16 1.44111 105 3 B₄C 1100 — 1231 0.71 0.23 0.81 77 71 4 TiC 950 H₂/H₂O 22371.23 0.61 1.11 116 110 5 SiC 1150 H₂/H₂O 1696 0.90 0.61 0.81 116 95 6Al₄C₃ 400 — 1204 0.81 0.55 0.63 104 91

[0046] In another embodiment it is also possible to react TiCl₄ with asuitable carbon source such as CH₄ to recycle the TiCl₄ back to TiC.

[0047] Titanium carbide may also be made by the reaction at hightemperature of titanium with carbon; titanium tetrachloride with organiccompounds such as methane, chloroform, or poly(vinyl chloride); titaniumdisulfide with carbon; organotitanates with carbon precursor polymersand titanium tetrachloride with hydrogen and carbon monoxide. Thereaction of titanium tetrachloride with a hydrocarbon-hydrogen mixtureat 1000° C. has been used in the prior art for the chemical vapordeposition (CVD) of thin carbide films used in wear-resistant coatings.

[0048] The SNC materials produced as described above are combined in aform suitable for use as a thin, flat, flexible electrode containinghigh fractions of the SNC. Theoretical models developed by the inventorspredict that the porous carbon electrodes should be essentially thin toprovide the high power output. Estimations show the optimum thickness tobe in the range of about 5-150 micron. Besides, one should bear in mindthat electrodes must not be brittle since they are normally pressed whenassembling the electrode pack in order to reduce the equivalent seriesresistance of a capacitor device.

[0049] According to the present invention, fabrication of compositeelectrodes of certain thickness may be accomplished by rolling aplasticized mixture of SNC powder, one or more binders and certainsolvents, said plasticized mixture being made as a stiff putty like masswith the help of the certain solvents.

[0050] Optional additives to the SNC carbon powder are colloidal orThermally Expanded Graphite(TEG) (1-15 % wt. of the dry mass) toincrease conductivity, conductive polymers (2-20 % wt. of the dry mass.)also to increase conductivity and SiO₂ (0.5-10% wt. of the dry mass)that increases capacitance.

[0051] After investigation of different types of compounds, which mightbe used as binders, fluorine-containing polymers, e.g. PTFE(Teflon) orPVDF poly(vinylidene fluoride) were selected as a permanent binder thatprovides structural integrity. The selection was based on the fact thatthose compounds keep both their binding properties and chemical andelectrochemical stability in electrolytes after the composite electrodematerial is thermally treated at temperatures below the decompositiontemperature of such polymers. In addition the carbon electrodesfabricated by the method do not lose their mechanical strength even ifthe binder content does not exceed 2-10% by wt. of the carbon powder.This results in relatively high capacitance and low resistivity of theEDLC comprising such electrodes.

[0052] Another incorporated binder is temporary and serves to facilitatethe formation of a ductile tape. It also raises the hydraulic componentof the rolling force during the roll compaction (below) and limits thecrushing force onto the carbon particles. The increased ductilityenables rolling to thinner sections without unduly stiffening orhardening of the product. The ductility also enables cross rolling ofthe extruded material which develops a more isotropic distribution ofTeflon fibers.

[0053] A requirement on this temporary binder is that it should becompletely removable, at a temperature below the decompositiontemperature of the permanent binder, without leaving any residues. Thishas the beneficial effect of leaving behind an improved transportporosity. One example of such temporary binder is polypropylenecarbonate (PPC). The proportion of this temporary binder should be 4-10%wt. of the carbon, preferably 5-7% wt. Before mixing, the binder isadded to a suitable solvent with the concentration of 7-18%, preferably10 %.

[0054] The solvent should have two advantageous characteristics. Itshould have low evaporation rate which means that the solvent contentshould change very slowly during material mixing and processing,resulting in better control and lot-to-lot reproducibility. Suchsolvents act as a plasticizer of the temporary binder and improves theworking range of the tape. If it did not act this way, then the additionof a special plasticizer would be required. Plasticizers do notthermally decompose in a manner similar to that of e g PPC and at thelow temperatures allowable would leave material behind. An additionalrequirement on the solvent is that it will evaporate completely, withoutleaving traces, along with the temporary binder. One such preferredsolvent is N-Methyl Pyrrolidone (NMP). The solvent is first added to thedry mix together with the dissolved temporary binder. Additional amountsof solvent is added during the appropriate processing steps until asuitable consistency of the mass is reached. The precise amount ofsolvent to be added depends on the type of carbon used, particularly onits specific surface.

[0055] For instance carbon made as indicated earlier from TiC, withoutsubsequent modification, requires that solvent be added until the ratioof the temporary binder to total solvent is 3-5%. Other carbon qualitiesmay require a higher or somewhat lower ratio of temporary binder tototal solvent content.

[0056] Although the method comprising extruding and rolling of thin andflexible carbon tapes by using binders such as PTFE is widespread, thepresent invention includes several improvements, that are necessary whenconsidering the SNC powder according to the present invention, to obtainsuperior electrochemical characteristics for electric double layercapacitors.

Process for Manufacturing Flexible Carbon Tapes for Electrodes

[0057] The method of making flexible carbon tapes can be by hand but itis more advantageously carried out in a series of mechanized steps thatlend enabling an integrated automated process.

[0058] The procedure for manufacturing flexible carbon tapes forelectrodes includes the following steps in order of sequence:

[0059] Dry mixing

[0060] Wet mixing

[0061] Muller mixing

[0062] Grinding

[0063] Extrusion

[0064] Roll compaction

[0065] Heat treatment

[0066] Alternatively we may proceed by

[0067] Dry mixing

[0068] Wet mixing

[0069] Roll mixing (Rubber Mill Processing)

[0070] Roll compaction

[0071] Heat treatment

[0072] Dry Mixing

[0073] Applicable methods are tumbling, ball milling or stirring ofchosen carbon powder, Teflon powder and optionally included additive asdescribed above.

[0074] Wet mixing

[0075] Wet mixing is a process to incorporate solvent such as NMP and asecondary binder such as PPC dissolved in a suitable solvent such asNMP. Wet mixing may be advantageously carried out in a planetary paddlemixer.

[0076] Muller mixing

[0077] Mulling is a process that effectively mixes the solid and liquidingredients and works the material into a soft, flexible mass. Thisoperation is performed in a bowl holding the components to be mixed anda cylinder inside the bowl located so that its outside surface is pushedby a spring against the inside surface and bottom of the bowl. Thematerial to be mixed is passed through the gap formed between the springloaded cylinder and the bowl wall. The material is contained within thebowl and is cycled back to the input of the process automatically. Dueto the pressure applied to the mixture during the mulling process, thereis extrusion of the material in both axes as the material is foldedback. This extrusion induces forces on the Teflon powder that stretch itinto fiber form.

[0078] Grinding

[0079] Grinding is a process in which the product from the muller is fedinto a system of rotating blades that cuts the material into smallpieces suitable for feeding into an extruder. The previous mixingprocess may have entrapped air in pockets in the material. The grindingfacilitates the removal of any such air when vacuum is applied to theextrusion hopper after loading the material into it.

[0080] Roll Mixing (Rubber Mill Processing)

[0081] This process is an alternative to mulling, grinding and extrusionto produce a belt preform suitable for roll compaction.

[0082] The step serves to further induce fiber formation of the Teflonportion of the binder system by stretching the Teflon particles. Theequipment and process conventionally used for mixing rubber compounds issuited to this requirement. The equipment consists of a pair of rollers,placed horizontally side by side so that the passage of material betweenthem will be vertical. The relative rotational speed of the rollers isset such that one roller turns faster than the other. The mixing isaccomplished by passing the materials through the rollers andcompressing it while simultaneously shearing it. This process isrepeated until the material is thoroughly blended. In order to start theprocess, the materials must be roughly blended together so they willform a mass that can be placed into the rolls. At the end of theprocess, the product is collected as a single belt perform suitable forroll compaction.

[0083] Extrusion

[0084] Extrusion is performed to produce a ductile belt preform,typically <1 mm thick, suitable for roll compaction.

[0085] Roll compaction

[0086] Roll compaction is a process in which a suitable ductile beltpreform is fed between rolls rotating at the same speed with the gap ornip set so that the resulting tape is of the desired thickness,typically about 100 micrometers.

[0087] The rolling action is predominantly a shearing process thatproduces the tape without unduly compressing it. The physical propertiesof the tape are influenced by several factors including the diameter ofthe rolls, the rolling speed and the reduction in thickness per pass.

[0088] Heat treatment

[0089] To remove without residual material traces the temporary binderand the solvent, the fabricated electrode is heat treated at atemperature that leaves the Teflon unaffected. The pyrolysis temperaturefor PPC is 250° C.

[0090] One preferred embodyment of the present invention provides anelectric double layer capacitor (EDLC), which comprises thin andflexible polarizable SNC electrodes providing both low internalresistance and high capacitance at the same time. This is achieved byfabricating thin composite electrodes having the thickness in the rangeof about 5-150 microns and being stable mechanically, chemically andelectrochemically in electrolytes over a long time. The electrodescomprise SNC carbon material as a powder, thermoexpanded graphite (TEG)as an additive, and a binder.

[0091] To fabricate electrodes in accordance with the present invention,nanoporous carbon materials produced by chlorinating titanium carbide,silicon carbide, molybdenum carbide, boron carbide, aluminum carbide ortheir combinations were used. These carbon materials possess areasonably large specific area (1000-2500 m²/g) including the notablecontribution from the pores of about 0.7-3 nm in size that enables theions from an electrolyte to enter the pores forming the electric doublelayer. The optimum carbon particle size in fabricated electrodesaccording to the present invention depends on the raw mass preparationmethod for the electrode sheet rolling but preferable are sizes notexceeding 10 micron. Powder having large grain size would cause poormechanical strength of the composite electrodes. Drawback of particlesexceeding 10 microns is also the increased resistivity of respectiveelectrodes caused by the limited rate of diffusion of ions inside theparticles.

[0092] Alternatively to the above method, a slurry of SNC carbon andother components as described above can be prepared, suitable for tapecasting or slurry rolling to yield continuous flexible thin tapes. Tapecasting could be made onto an aluminum foil or mesh so that thisaluminum current collector can be directly incorporated into theelectrode in a single manufacturing step.

[0093] To reduce the internal resistance of a EDLC device, in accordancewith the present invention an aluminum layer of 2-5 microns thicknessmay be deposited on one side of composite electrodes by using anappropriate deposition method such as Plasma Activated Physical VaporDeposition. The contact between the composite electrodes and aluminumfoil or mesh (the current collector) is provided by pressing themtogether, by diffusion welding, spot or seam welding or laser welding.

[0094] Magnetic pulse welding or joining is another method with theadvantage of being a “cold process”.

[0095] All types of electrolytes used in electric double layercapacitors may be used for the present invention, water based (e g KOH,H₂SO₄) and organic. The non-aqueous electrolytic solution preferablycomprises at least one salt selected from the group oftetrafluoroborates or hexafluorophosphates of tetraalkylammonium,tetrakis (dialkylamino) phosphonium,N,N-dialkyl-1,4-diazabicyclo[2.2.2]octanediium or their mixture,dissolved in an aprotic polar solvent or a mixture of such solventsselected from the group consisting of acetonitrile, propionitrile,benzonitrile, butyronitrile, 3-methoxypropionitrile,gamma-butyrolactone, -valerolactone, ethylene carbonate, propylenecarbonate, N,N-dimethylformamide, 1-methyl-2-pyrrolidinone,dimethoxyethane, methyl ethyl ketone and tetrahydrofuran. The generalrequirement of useable electrolytes are the chemical and electrochemicalstability and good performance over a wide temperature range. In orderto avoid electrolyte depletion between the electrodes of the EDLC, thetotal salt concentration in the non-aqueous electrolyte is chosen in therange of 0.5-3 mol/l according to the present invention.

[0096] Organic electrolytes are widely used to increase voltage, andhence, specific performances of an electric double layer capacitor andare preferred for high energy applications. However, most of knownelectrolytes comprise cations and anions of different size. In manycases, large organic cations cannot enter small pores resulting in muchlower capacitance of the negative electrode, and hence of the entirecapacitor device. To provide an electric double layer capacitor, thepresent invention aims at using unsymmetrical polarizable electrodes inorder to increase both the capacitance and voltage of an electric doublelayer capacitor resulting in its higher specific energy and power.

[0097] Equalizing Leakage Current

[0098] Unit cells can be manufactured and selected so that theircapacitance and inner resistance are practically equal along a stack ofseries connected EDCLs, however, it is rather difficult to equalizetheir leakage current. Even a small deviation in the leakage currentvalue for various unit cells along the SC stack can cause a significantdeviation from mean voltage value after keeping the charged stack forsome time. In its turn, the disbalance in voltage can cause thedecomposition of electrolyte in the cells charged up to a voltage higherthan their rated voltage during further cycling the stack.

[0099] To improve a supercapacitor performance and to equalize thevoltage of unit cells when they are assembled in a stack, some additivescan be added to the electrolyte. As another embodiment, the presentinvention discloses a number of compounds, which undergo a fullyreversible electrochemical reaction within a potential range not farfrom that wherein the impurities in the electrolyte start decomposing.Said compounds are chosen from aromatic series, the preferable compoundsbeing twinned aromatic hydrocarbons (including heterosubstituted ones),aromatic nitrites, quinones and nitro- or amino-derivatives. Suchcombinations as nitronitriles (nitro-cyano derivatives) orcyanosubstituted quinones can also be used. All these compounds possessat least one reversible electrochemical wave either in anode or incathode region (or in both regions), said electrochemical wave beinglocated not far from the potential, at which the electrolytedecomposition starts. The concentration range wherein said additives areeffective enough to influence the electrode potential and leakagecurrent without deteriorating the performance of a supercapacitor deviceis between 1×10⁻⁴ and 1×10⁻¹ mol/l, preferably between 1×10⁻³ and 1×10⁻²mol/l. The electrochemical characteristics of some selected compoundsare presented in Table 4. TABLE 4 Electrochemical characteristics ofsome compounds selected as voltage equalizing additives Anodic processCathodic process No. Compound E_(p), V^(a) E_(p), V¹ E_(p), mV 1Anthracene ≈0.9 −2.21 60 2 1,2- — −2.025 70 Dicyanobenzene 35-Nitro-1,2- — −1.09 60 dicyanobenzene −1.685 90 4 1-Cyano- 1.75 −2.3460 naphthalene 5 Anthraquinone — −1.18 80 −2.83 60

[0100] 2.2 V (0.5 mA/cm² was chosen as a limiting current density).Reduction of water impurities starts at ca. −2.3 V.

[0101] The electric double layer capacitor includes a porous,ion-permeable, insulating material (separator) interposed betweenelectrodes. It may be selected from the group of a nonwovenpolypropylene or polyethylene separator films, a cellulose separatorpaper, a polyethylene terephthalate nuclear membrane; the separatorthickness being about 5-100 micron, preferably 5-20 micron. The standardseparator used in the art are PP based microporous separator films fromCelgard GmbH (Germany).

[0102] Alternatively dielectric materials (such as SiO₂, SiCN or Al₂O₃)may be deposited as a thin film (of 0.1-3 microns) on the electrodesurface. Our experiments show that sputtering a thin porous dielectricfilm improves both the mechanical properties and electrical performanceof the composite electrodes.

[0103] Yet another method to provide a separator is to use a screenablepaste permeable membrane compound formed from a silicon oxide aerosolcarried in a PVDF/NMP paste. The dried film properties are controlled bythe ratio of the SiO2 wt. to the resin wt. and the dried film thicknessto the solvent percent.

[0104] In another aspect of the invention, balancing of the positive andnegative electrodes is provided. One carbon was chosen for the anode andanother for the cathode to match the sizes of the positive and negativeions of the electrolyte.

[0105] Of particular advantage, for a given electrolyte we estimate thesizes of the ions and then choose the appropriate carbon precursor andprocess parameters which gives us the SNC with the matching porecharacteristics. FIG. 7 illustrates a variety of electrolytic saltssuitable for use in the present invention. Sources of information aboutions sizes are e g

[0106] 1. Makoto Ue. J. Electrochem. Soc., (1994) vol. 141, No. 12, p.3336

[0107] 2. Makoto Ue. Electrochim. Acta, (1994) vol. 39, No.13, p. 2083.

[0108] Both crystallographic data and MM2 calculations were used toestimate the ion size (van der Waals volume and radii) for a number oftetraalkylammonium cations as well as for some anions and solventmolecules.

[0109] For doubly-charged N,N-dialkyl-1,4-diazabicyclo[2.2.2]octanediium(DEDACO²⁺) cation, the size was estimated by the inventors from the sizeof fragments included.

[0110] In one example, a positive polarizable electrode is made ofnanoporous carbon material having an surface area of 1500 m²/g accordingto BET measurements and a pore size of 0.5-1.5 nm preferably 0.5-1.0 nm.The negative polarizable electrode is made of carbon material having anaverage surface area of 2000 m²/g and a pore size of 1.0-3.0 nm,preferably 1.0-2.0 nm.

[0111] In another aspect of the invention, a supercapacitor is providedwhere the specific capacitance of the cathode and the anode aredifferent. If electrodes of the same size are used then the one having alower capacitance, determines the cell as a whole by this lowercapacitance level. To compensate for this we increase the volume(thickness) of the electrode (cathode) to raise the capacitance to thatof the anode. The positive and negative capacitance need to be the samefor most efficient energy storage.

[0112] In yet another aspect of the invention, a supercapacitor isprovided where the positively and negatively charged electrodes in anelectrode pair are balanced according to the zero-charge potential ofthe chosen electrode material. Balancing the electrodes with respect ofthe amount of stored charge considering the electrode's zero-chargepotential and the applicable electrochemical window (i.e. the region ofan ideal polarizability) increases the nominal voltage andelectrochemical stability of a capacitor.

[0113] All supercapacitors contain three key components: electrodes,separator and electrolyte. It is the interdependent tuning of theproperties of these elements that is necessary for and contribute to thehigh performance.

[0114] Design of the cell is important. If the electrode is thin, boththe current collector and separator have to be thin. Balancing ofdissimilar electrodes is very important and again hinges on our abilityto tailor make the pore size by choosing the appropriate carbonprecursor and on the processing and post processing operations. Thisfeature also applies to the ability to match electrolyte ion size andpore size. A further important feature is to adjust the size ofelectrodes so that they deliver the same capacitance. The combination ofthese features gives the high performance of the supercapacitors of thisinvention.

[0115] The ability to control the nanopores size and its distribution isof course also beneficial for supercapacitors based on aqueouselectrolyte systems( e g KOH, H_(S)SO₄). Table 5 shows that thecapacitance in such a system can be influenced by choice of precursormaterial and thus the pore characteristics. Further refinement bymodification of this nanoporosity offers optimization opportunities.TABLE 5 Examples of influence on capacitance of choice of carbonprecursors. W_(s) S_(a,BET) Capacitance* # SNC precursor [cm³g⁻¹][m²g⁻¹] [F g⁻¹] 1 Al₄C₃ 0.60 1353 251 2 B₄C 0.78 1782 217 3 Mo₂C 0.901873 223 4 TiC 0.73 1340 212 5 SiC 0.44 1059 209 6 TiC/Al₄C₃, {fraction(3/1)} (by wt.) 0.58 1542 239 7 B₄C/Al₄C₃, {fraction (3/1)} (by wt.)0.67 1614 239 8 B₄C/Al₄C₃, {fraction (1/1)} (by wt.) 0.58 1572 219 9B₄C/Al₄C₃, ⅓ (by wt.) 0.53 1440 211 # cell with Hg/HgO referenceelectrode

[0116] The foregoing description of specific embodiments and examples ofthe invention have been presented for the purpose of illustration anddescription, and although the invention has been illustrated by certainof the preceding examples, it is not to be construed as being limitedthereby. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and obviously manymodifications, embodiments, and variations are possible in light of theabove teaching. It is intended that the scope of the invention encompassthe generic area as herein disclosed, and by the claims appended heretoand their equivalents.

EXAMPLE 1

[0117] Preparation of Carbon from TiC in Rotary Kiln Reactor

[0118] Titanium carbide (H. C. Starck, grade C.A., 300 g) with anaverage particle size of 1.3-3 microns was loaded into the silica rotarykiln reactor and let to react with a flow of chlorine gas (99.999%assay) for 4 h in a tube furnace at 950° C. Flow rate of chlorine gaswas 1.6 l/min and rotation speed of reactor tube was ˜2.5 rpm. Theby-product, TiCl₄, was led away by the stream of the excess chlorine andpassed through the water-cooled condenser into the collector. After thatthe reactor was flushed with the Argon (0.5 l/min) at 1000° C. for 0.5 hto remove the excess of chlorine and residues of a gaseous by-productsfrom carbon. During heating and cooling, the reactor was flushed with aslow stream (0.5 l/min) of argon. Resulting carbon powder (47.6 g) wasmoved into silica stationary bed reactor and treated with hydrogen gasat 800° C. for 2.5 h. During heating and cooling, the reactor wasflushed with a slow stream of Helium (0.3 l/min). Final yield of thecarbon material was 45.6 g (75.9% from theoretical).

EXAMPLE 2

[0119] Preparation of Carbon from TiC in Fluidized Bed Reactor

[0120] Titanium carbide (Pacific Particulate Materials, 1.0 kg) with anaverage particle size of 70 microns was loaded into the silica fluidisedbed reactor and let to react with a flow of chlorine gas (99.999% assay)for 4 h at 950° C. Flow rate of chlorine gas was 7.5 l/min. Theby-product, TiCl₄, was led away by the stream of the excess chlorine andpassed through the water-cooled condenser into the collector. After thatthe reactor was flushed with the Argon (6 l/min) at 1000° C. for 0.5 hto remove the excess of chlorine and residues of a gaseous by-productsfrom carbon. During heating and cooling, the reactor was flushed with aslow stream (0.5 l/min) of argon. Final yield of the carbon material was190 g (95% from theoretical).

EXAMPLE 3

[0121] Preparation of Carbon from Mo₂C in Stationary Bed Reactor

[0122] Molybdenum carbide (Donetsk Chemical Reagent Plant JSC, Ukraine,Lot TY6-09-03-363-78, particle size <40 micron, 100 g) The molybdencarbide was loaded into the silica stationary bed reactor and allowed toreact with a flow of chlorine gas (99.999% assay) for 80 min. in a tubefurnace at 750° C. Flow rate of chlorine gas was 1.6 l/min. Theby-product, mixture of molybdenum chlorides, was led away by the streamof the excess chlorine and passed through the water-cooled condenserinto the collector. After that the reactor was flushed with the Argon(0.5 l/min) at 1000° C. for 0.5 h to remove the excess of chlorine andresidues of a gaseous by-products from carbon. During heating andcooling, the reactor was flushed with a slow stream of argon. Resultingcarbon powder (4.9 g) was moved into silica stationary bed reactor andtreated with hydrogen gas at 800° C. for 1 h. During heating andcooling, the reactor was flushed with a slow stream of helium (˜0.3l/min). Final yield of the carbon material was 4.6 g (78% fromtheoretical).

EXAMPLE 4

[0123] Modification of SNC of Example 1 in Stationary Bed Reactor andFluidized Bed Reactor

[0124] A carbon powder of Example 1 (25 g) was placed in a quartzreaction vessel and loaded into horizontal quartz reactor heated by thetube furnace. Thereupon the reactor was flushed with argon to remove airand the furnace was heated up to 900° C. using a heat-up gradient of15°/min. The argon flow was then passed with a flow rate of 0.8 l/minthrough the distilled water heated up to 75-80° C. and the resultantargon/water vapor mixture with approximate ratio of 10/9 by volume waslet to interact with a carbon at 900° C. for 2.5 h. After that thereactor was flushed with argon for one more hour at 900° C. to completethe activation of a carbon surface and then slowly cooled to roomtemperature. The yield of a modified carbon was 15.8 g.

EXAMPLE 5

[0125] Preparation of Carbon from SiC in Rotary Kiln Reactor withSubsequent Modification in a Stationary Bed

[0126] Silicon carbide (H. C. Starck, lot 3481, particle size <10micron, 100 g) was loaded into the silica rotary kiln reactor andallowed to react with a flow of chlorine gas (99.999% assay) for 1 h ina tube furnace at 1150° C. Flow rate of chlorine gas was 1.5 l/min androtation speed of reactor tube was ˜2.5 rpm. The by-product, SiCl₄, wasled away by the stream of the excess chlorine and passed through thewater-cooled condenser into the collector. After that the reactor wasflushed with the Argon (0.5 l/min) at 1150° C. for 0.5 h to remove theexcess of chlorine and residues of a gaseous by-products from carbon.During heating and cooling, the reactor was flushed with a slow stream(0.5 l/min) of argon. Resulting carbon powder (29.9 g) was moved intosilica stationary bed reactor and treated with hydrogen gas at 800° C.for 2 h. During heating and cooling, the reactor was flushed with a slowstream of helium (˜0.3 l/min). The yield of the carbon material was 28.7g (95.6% from theoretical). Part of a carbon powder (15 g) obtained wasplaced in a quartz reaction vessel and loaded into horizontal quartzreactor heated by the tube furnace. Thereupon the reactor was flushedwith argon to remove air and the furnace was heated up to 900° C. usinga heat-up gradient of 15°/min. The argon flow was then passed with aflow rate of 0.8 L/min through the distilled water heated up to 75-80°C. and the resultant argon/water vapor mixture with approximate ratio of10/9 by volume was let to interact with a carbon at 900° C. for 2.5 h.After that the reactor was flushed with argon for one more hour at 900°C. to complete the modification of a carbon surface and then slowlycooled to room temperature.

EXAMPLE 6

[0127] Experimenetal Laboratory Preparation of Electrode

[0128] A mixture including 86% wt of SNC powder of Example 1 and 8% wtof TEG was stirred in ethanol for 10 minutes. After that 6% wt of PTFE(as a suspension in water) was added to this slurry preliminary cooledto 15° C., stirred for 30 minutes and gently pressed until the wet“pancake” was formed. Thereupon the ethanol was evaporated at elevatedtemperature (˜95° C.). This pancake was then impregnated with heptane,shaped to a cylinder and extruded by rolling the body in direction ofthe ends of a cylinder. The latter procedure was repeated until theelastic properties appeared. Finally the heptane was removed at ˜75°,the extruded cake rolled stepwise down to thickness of 98±4 microns,dried in vacuum at 270° C. and covered from one side with an aluminumlayer of 4±1 μm using Plasma Activated Physical Vapor Deposition.

EXAMPLES 7-10

[0129] Further Electrodes Preparation

[0130] Polarizable electrodes were made in the same manner as in example6 except that SNC powder from Examples 2-5 was used, respectively.Carbon powder with particle sizes >10 micron were reduced by ballmilling. The electrode sheets were prepared with a thickness of 98±4,125±5, 125±5 and 125±5 microns for the SNC of examples 2, 3, 4 and 5,respectively.

[0131] Assembling and Preconditioning of Capacitors from the Electrodeof Examples 6-10

[0132] The electrodes as prepared according examples 6-10 were attachedby methods indicated in TABLE 5 to Al foil of 10 microns thick (currentcollector) and interleaved with a separator. A Celgard separator wasused in the present examples The electrode pairs from positively andnegatively charged polarizable electrodes made as disclosed in Examples6-10, were connected in parallel. The electrode pack thus prepared wasplaced in a sealed box, kept under vacuum for three days to remove allthe gases absorbed and then impregnated with electrolyte comprisingsolution of a single quaternary ammonium salt or a mixture of such inacetonitrile. The EDLC cells thus fabricated were cycled within thevoltage range of 1.5-3.0 V under constant current or constant powerconditions.

[0133] Certain of the results obtained are shown in Tables 6 and 7: Theapplicability of different methods to connect the carbonaceous electrodesheet to the aluminum foil is presented in Table 6 and the effect ofbalancing the positively and negatively charged polarizable electrodesis presented in Table 7. TABLE 6^(a) Results of different methods ofconnecting an aluminum current collector to a carbonaceous electrodesheet. Volumetric Type of connection Resistance Capacitance Capacitancebetween the electrode Electrodes (active (Per active (Per active SNCused in +/− and the current thickness Resistance volume)^(b) weight)^(b)volume)^(b) electrodes collector (+/−) [Ωcm²] [Ωcm³] [F g⁻¹] [F cm⁻³]C(TiC)/C(TiC)- Electrode/Al foil 100/120 0.87 0.037 6.7 9.1 modified(arc spot weld) C(TiC)/C(Mo₂C) Electrode/Al foil 102/120 0.60 0.023 6.79.7 (diffusion weld) C(TiC)/C(TiC)- Electrode/Al foil 95/130 0.69 0.0256.9 10.7 modified (pressure contact) C(TiC)/C(TiC)- Electrode/Al foil100/130 0.68 0.027 6.7 10.0 modified (laser spot-weld)

[0134] TABLE 7 Examples of the effect of balancing positively andnegatively charges polarizable electrodes Volumetric ResistanceCapacitance Capacitance (active (Per active (Per active Electrod SNCused in Resistance volume) weight)^(a) volume)^(a) Electrolyte C_(salt)Thicknes +/− electrodes [Ωcm²] [Ωcm³] [F g⁻¹] [F cm⁻³] Separator salt[M] +/− C(Mo₂C)/ 0.42 0.017 4.2 6.7 Celgard 277 Et₄NBF₄ 1.5 130/130C(Mo₂C) C(TiC)/ 0.29 0.010 7.8 12.2 Celgard 277 Et₄NBF₄ 1.5 105/135C(TiC)- modified C(TiC)/ 0.38 0.014 6.0 9.4 Celgard 277 Et₄NBF_(4/)0.745/  98/128 C(Mo₂C) Et₂Me₂NBF₄ 0.846 C(TiC)/ 0.47 0.015 6.9 9.0Celgard 2400 Et₄NBF₄ 1.5 140/140 C(TiC) C(B₄C)/ 0.61 0.018 5.2 6.8Celgard 2400 Et₄NBF₄ 1.5 115/115 C(B₄C)

[0135] The examples of capacitor prototypes and their electrochemicalperformance illustrating the object of this invention are presented inTable 8 and FIG. 6. TABLE 8 Examples of electrochemical performance ofprototype electric double layer capacitors according to the presentinvention. Specific Specific Capacitance Resistance capacitanceresistance # Electrolyte [F] [mΩ] [F g⁻¹] [F cm⁻³] [Ωcm²] [Ωcm³] 1 1.5 MTEA/AN 630 0.56 7.39 9.63 0.87 0.037 2 0.75 M TEA + 663 0.68 7.47 9.931.16 0.045 0.75 M TEMA/AN 3 1.0 M TEA/AN 631 0.54 7.35 9.54 0.84 0.036

[0136] Characterization of SNC Materials According to this Invention

[0137] The low temperature nitrogen sorption experiments were performedat the boiling temperature of nitrogen (−196° C.) using GeminiSorptometer 2375 (Micromeritics). The specific surface area of carbonmaterials was calculated according BET theory up to the nitrogenrelative pressure (p/p₀) of 0.2, with the exception for results reportedin Table 1. The volume of micro-pores was calculated from the t-plot ofadsorption isotherm and the pore size distribution according toBJH(Barrett-Joyner-Halenda) theory. Adsorption dynamics of benzenevapours was studied at room temperature using the computer controlledweighing of the carbon samples in benzene vapours at normal pressure androom temperature. A volume of the pores that adsorbed benzene inabove-described conditions, was calculated according the equation

W _(s)=(m ₂ −m ₁)/m ₁ ×d _(C) ₆ _(H) ₆ [cm³g⁻¹]

[0138] where m₁ and m₂ are the initial and final weights of thetest-sample, respectively, and d_(C) ₆ _(H) ₆ is the density of benzeneat room temperature.

[0139] Electrochemical Evaluation of SNC Materials

[0140] The electrochemical tests were performed in the 3-electrodeelectrochemical cell, using the Solartron potentiostat 1287 with FRAanalyzer. Electrochemical experiments were done in aqueous, 6M KOH andnon-aqueous 1.5M Tetraethylammoniumtetrafluoroborate (TEA) inAcetonitrile (AN) electrolyte. During experiments the electrolyte wasdegassed with Ar gas.

[0141] Three types of experiments using: constant voltage (CV), constantcurrent (CC), and impedance (EIS) technique were used. The region of theideal polarizabilty was observed between −1.5 to +1.5V (vs. SCE) and−1.0 to +0.25V (vs. Hg/HgO) for non-aqueous and for aqueous systems,respectively. Discharge capacitance for the negatively and positivelycharged electrode materials were calculated from the CV and CC plots.The EIS measurements were carried out at constant DC potentials: −1.4V,+1.4V for non-aqueous and −1.0V, in aqueous electrolytes. The EIScapacitance was calculated at frequency 10 mHz.

[0142] Evaluation of Supercapacitors

[0143] The constant current (CC) and constant voltage (CV) tests werecarried out using the potentiostat Solartron 1287. The nominal voltageof capacitors was estimated from the CV plots. The capacitance of thesupercapacitors was calculated from CC plots according to formula:C=Idt/dE. Internal resistance was derived from the IR-drop.

[0144] The power, energy performance and respective Ragone plots werecharacterised, using constant power (CP) charge-discharge cyclingregimes.

[0145] Electrochemical impedance spectroscopy (EIS) was used todetermine series capacitance and series resistance at frequencies 10 mHzand 100 Hz, respectively.

[0146] In summary, the present invention provides superiorsupercapacitor performance. Specifically, our SNC has a combination ofhigh specific surface area and narrow pore size distribution in a highpacking density of the electrode, which is better than any other knowncarbon. This allows the making of very thin electrodes, which providethe low resistance and the high power of the device, while stillmaintaining high specific energy. Balancing the electrochemicalperformance of positively and negatively charged electrodes by varyingtheir composition and volume, in accordance with the objective of thepresent invention, is also a key feature of the method for manufacturingthe EDLC proposed.

1. An electric double layer capacitor including at least one pair ofpolarizable electrodes connected to current collectors, a separator madeof ion-permeable but electron-insulating material interposed between theelectrodes in each pair of electrodes, and a liquid electrolyte,characterised in that the electrodes include a layer of carbon particleshaving a narrow distribution of nanopores therein, the pore sizes of thenanopores being adapted to fit the ion sizes of the electrolyte.
 2. Thecapacitor according to claim 1, characterised in that the layer ofcarbon particles in each electrode includes 2-10% wt of a binder.
 3. Thecapacitor according to claim 2, characterised in that the binder is afluorine-containing polymer.
 4. The capacitor according to claim 3,characterised in that the layer of carbon particles in each electrodeincludes 1-15 wt % of thermo-expanded graphite (TEG) for improving thetransport of ions in the layer.
 5. The capacitor according to claim 3,characterised in that the layer of carbon particles in each electrodeincludes 1-15 wt. % of collodial carbon for improving the transport ofions in the layer.
 6. The capacitor according to claim 1, characterisedin that the layer of carbon particles in each electrode includes 0.5-10wt. % of SiO₂ for increasing capacitance.
 7. The capacitor according toclaim 1, characterised in that the thickness of the electrodes lieswithin the range of 5-150 microns. 8 The capacitor according to claim 7,characterised in that the size of the carbon particles in the electrodesis less than about 10 microns.
 9. The capacitor according to claim 1,characterised in that the carbon particles in the electrodes areproduced by halogenation of particles of inorganic carbon containingcompounds.
 10. The capacitor according to claim 9, characterised in thatthe carbon particles in the electrodes are produced by halogenation ofparticles of inorganic carbon containing compounds based on metals,metalloids or combinations thereof from the group of Ti, Zr, Hf, V, Nb,Ta, Mo, W, Cr, Fe, Al, Si, B and Ca.
 11. The capacitor according toclaim 9, characterised in that the nanopores in the electrodes of eachpair of electrodes have different sizes in order to match differentsizes of anions and cations in the electrolyte.
 12. The capacitoraccording to claim 11, characterised in that the electrodes in each pairof electrodes have the same capacitance.
 13. The capacitor according toclaim 12, characterised in that the electrodes in each pair ofelectrodes have different volume.
 14. The capacitor according to claim13, characterised in that the positively and negatively chargedelectrode in each pair of electrodes are balanced according tozero-charged potential.
 15. The capacitor according to claim 11,characterised in that a layer of aluminium having a thickness of 2-5microns is deposited on the side of the electrode which is attached to acurrent collector.
 16. The capacitor according to claim 1, characterisedin that the separator is a porous dielectric film or paper, such asanonwoven polypropylene, a polyethylene separator film, a polyethyleneterephthalate nuclear membrane or a cellulose separator paper, theseparator thickness being about 5-100 microns, preferably 5-30 microns17. The capacitor according to claim 1, characterised in that theseparator is a thin film of a dielectric material deposited on eachelectrode on the side opposite to the side to which the currentcollector is attached.
 18. The capacitor according to claim 1,characterised in that the liquid electrolyte is a water basedelectrolyte or an organic electrolyte.
 19. The capacitor according toclaim 18, characterised in that the liquid electrolyte comprises atleast one salt selected from the group of tetrafluorborates orhexafluorophosphates of tetraalkylammonium, tetrakis(dialkylamino)phosphonium, N,N-dialkyl-1,4-diazabicyclo[2.2.2]octanediium or theirmixture, dissolved in an aprotic polar solvent or a mixture of suchsolvents selected from the group of acetonitrile, propionitrile,benzonitrile, butyronitrile, 3-methoxypropionitrile,gamma-butyrolactone, -valerolactone, ethylene carbonate, propylenecarbonate, N,N-dimethylformamide, 1-methyl-2-pyrrolidinone,dimethoxyethane, methyl ethyl ketone and tetrahydrofuran; theconcentration of salts being 0.5-3.0 mol/l.
 20. An electrolyte for anelectric double layer capacitor, characterised in that a voltagelevelling agent, chosen from the groups of twinned aromatichydrocarbons(including heterosubstituted ones), aromatic nitrites,quinones and nitro- or amino-derivatives or combinations ofnitronitriles (nitro-cyano derivatives) or cyanosustituted quinones iscomprised in the electrolyte.
 21. A method of manufacturing carbonparticles with a predetermined pore size, characterised by the step ofhalogenation of particles of an inorganic carbon containing compound,whereby nanopores of a given size are created in the carbon particlesremaining after halogenation, and selecting the carbon containingcompound so that the nanopores in said carbon particles have a size thatis somewhat smaller than or equal to the predetermined size, and, ifneeded, by the further step of treating said carbon particles in anoxidising medium until the nanopore size in said carbon particlescorrespond to the predetermined size.
 22. The method according to claim21, characterised in that the halogenation is performed in a fluidisedbed or in a rotary kiln reactor.
 23. The method according to claim 21,characterised in that H₂O, HNO₃, H₂SO₄, HCl, NaOCl or CO₂ is used tooxidize the carbon particles remaining after halogenation.
 24. A methodof manufacturing an electrode for an electric double layer capacitor,characterised by mixing a mixture comprising carbon particles havingnanopores with a predetermined size, at least a primary binder and asecondary binder and a solvent, extruding the mixture, rolling theextruded mixture into sheet shape and thereafter attaching the formedelectrode sheet to a conductive foil or mesh.
 25. The method accordingto claim 24, characterised in that the mixing step comprises dry mixing,wet mixing, muller mixing and grinding.
 26. A method of manufacturing anelectrode for an electric double layer capacitor, characterised bymixing a mixture comprising carbon particles having nanopores with apredetermined size, at least a primary binder and a secondary binder anda solvent, roll mixing the mixture, rolling the mixture into sheet shapeand thereafter attaching the formed electrode sheet to a conductive foilor mesh.
 27. The method according to claim 26, characterised in that themixing step comprises dry mixing and wet mixing.
 28. The methodaccording to claim 24, characterised in that the step of rolling themixture into sheet shape is performed by roll compaction.
 29. The methodaccording to claim 28, characterised by removing the solvent and thesecondary binder from the formed electrode sheet by heat treatment, saidsecondary binder being removable from the formed electrode sheet at atemperature below the decomposition temperature of the first binder. 30.The method according to claim 29, characterised in that the first binderconsists of 2-10 wt % of particles of polytetrafluorethylene orpolyvinylidene fluoride and in that the mixing is performed so that theparticles of polytetrafluorethylene or polyvinylidene fluoride arepulled into filaments.
 31. The method according to claim 29,characterised in that the second binder has the ability to facilitatethe formation of a ductile tape and raise the hydraulic component of therolling force and is removable at a temperature below the decompositionof the first binder.
 32. The method according to claim 29, characterisedin that a solvent, which has a low evaporation rate and the ability ofplasticizing the secondary binder and which is removable at atemperature not exceeding the temperature at which the secondary binderis removable, is used.
 33. The method according to claim 32,characterised in that N-Methyl Pyrrolidone is used as a solvent.
 34. Themethod according to claim 31, characterised in that 5-15 wt %.polypropylene carbonate dissolved in the solvent is used as a secondarybinder.
 35. The method according to claim 34, characterised by heattreating the formed electrode sheet at a temperature of about 250° C. sothat the polypropylene carbonate is pyrolysed.
 36. The method accordingto claim 24, characterised in that 1-15 wt % of thermo-expanded graphite(TEG) is comprised in the mixture.
 37. The method according to claim 24,characterised by attaching the formed electrode sheet to a conductivefoil or mesh by pressing, diffusion welding, spot or seam welding orlaser welding.
 38. Use of magnetic pulse joining for attaching a foil ormesh of conductive material to a sheet of carbonaceous electrode.
 39. Amethod of manufacturing an electrode for an electric double layercapacitor, characterised by mixing a mixture comprising carbon particleshaving nanopores with a predetermined size, a binder and a solvent to aslurry and tape casting the slurry directly onto a conductive foil. 40.The method according to claim 24, characterised by depositing a thinfilm of a dielectric material on each electrode on the side opposite tothe side to which the conductive foil or mesh is attached.
 41. Themethod according to any one of claim 24, characterised by depositingsilicon oxide aerosol carried in a paste of PVDF/NMP on each electrodeon the side opposite to the side to which the conductive foil or mesh isattached by screening and drying the paste into a film.